Hi I attended the Air Assault School way back in 1989 - Slingload was a great experience and i took a few photos
BTW this is Camp Gruber OK—where many Deutsches Afrikakorps POWs sat out the war
Pretty sure you’re looking at an A or B model Chinook. It could be a C model, because you couldn’t tell the three apart from looking at them. The difference between A and B is a redesigned rotor stability system and non-symmetrical rotor blades. The C model’s only real difference is an improved and more powerful engine with increased GVW limit.
The D Model came with a three hook system, more advanced engines and composite rotor blades. The only physical difference you would see would be the three hooks underneath instead of the one. The D Model was introduced in 1982. In 1989, a D model would have been a front line unit, not one used for training (though it’s possible, most advanced models tended to be sent where they’d be most useful – up close to where they’d be most needed.)
At best, I’d think this could possibly be a C model, though more than likely it would have been an older A or B unit. Again, you’d have to know by tail number and production line, because visually the A-D models look exactly alike with the exception of the D having the 3 Hook system undercarraige points.
The non-symmetrical rotor blades cannot be told while they are in operation visually, the difference between them and the previous symmetrical rotor blades is determined by the shape of the upper and lower surfaces of the blades themselves. On non-symmetrical systems the upper portion of the blade is curved to provide more lift than the earlier symmetrical models which were flat on both surfaces (relatively speaking.)
Thanks dunniteowl - I really appreciate your analysis—It fits in as well for the school to be using Vietnam era equipment—Rappelling was with Hueys, but we did get a Blackhawk for medevac training
The amount of expertise on this forum is pretty exceptional. I’ve done it myself more than once — post a picture, basically say, “Hey, can anybody help me figure this out?” and before long somebody with firsthand knowledge of the subject shows up with a detailed answer.
I learned something too. I had no idea about the differences between the A, B, C and D Chinooks before reading this, and now I have a much better understanding of how they evolved.
Modelers helping modelers. This place is pretty cool sometimes. 
